Can a player play a Reaction speed card (like Shen) during a Showdown to defend against an attack, even though the Reaction isn't responding to a specific spell or ability?

Ruling: Yes, Reaction speed cards can be played during a Showdown. Whenever you can play an Action speed card, you can also play a Reaction speed card.

Sequence:

• When contesting an empty battlefield or a battlefield controlled by the opponent, a Showdown occurs
• During a Showdown, players can play Action cards while there are no effects on the chain waiting to resolve
• Since Reaction cards have all the advantages of Action cards, they can also be played during this window
• The defender could play Shen as a reaction to the attack, forcing the attack to hit Shen first

Nuances:

• Action speed has all the advantages of base speed
• Reaction speed has all the advantages of Action speed and base speed
• Whenever you can play base speed cards, you can play Action or Reaction speed cards